Between 8-12% of people exposed to opioids develop opioid use disorder (OUD). Developing more effective preventions and treatments for OUD requires a better understanding of genomic and epigenetic mechanisms that underlie individual vulnerability to distinct stages along the OUD trajectory (e.g., initial use, compulsive use, relapse). The overall goal of this proposal is to use an outbred rat model (Sprague-Dawley) to identify novel downstream genes and upstream regulators of gene transcription involved in 3 behavioral phenotypes associated with distinct stages along the OUD trajectory. By comparing rats that show high versus low levels of addiction-like behavior at each stage, we will be able to identify changes in gene expression and their regulation associated specifically with susceptibility to opioid addiction from among the numerous effects of opioids that are unrelated to addiction. Anhedonia produced during withdrawal from acute morphine (i.e., withdrawal-induced anhedonia, WIA) will be used as an addiction phenotype of the earliest phase of OUD, i.e. prior to voluntary drug consumption. We have previously found that WIA is more strongly associated with a range of measures of subsequent i.v. morphine self-administration (SA) than these SA measures are with one another. Economic demand for morphine and reinstatement after extinction will serve as measures of drug reinforcement efficacy and propensity for relapse, respectively, after extensive morphine SA. To identify vulnerability-related genomic targets, we will use Next-Generation Sequencing (NGS) techniques and advanced bioinformatic tools to compare transcriptomic and epigenomic differences in rats exhibiting high versus low levels of WIA (Aim 1), demand (Aim 2) or reinstatement (Aim 3). Our epigenomic assays will map loci of chromosomal accessibility (using ATAC-seq) and of the stable chromatin mark H3K4me3 (using ChIP- seq). We will overlay each of these data sets onto RNA-seq data to identify genes showing differential activation in High- versus Low-Susceptibility rats, as well as upstream regulators of these transcriptional effects. These assays will focus on the medial prefrontal cortex (mPFC), a node within the mesocorticolimbic system that plays a pivotal role in addiction. We will also overlay epigenomic maps derived from our studies onto genotyping data derived from larger studies of Heterogeneous Stock (HS) rats manifesting High- versus Low addiction-related behavioral phenotypes (e.g., drug intake, impulsivity), in other NIDA Animal Genetics Consortium U01/P50 projects. We hypothesize that these comparisons will yield a set of downstream genes and upstream regulators associated with individual differences in early vulnerability to addiction-like behavior and its severity once established. We further hypothesize that our transcriptomic and epigenomic data will provide a viable roadmap for identifying genetic variants from larger genomic datasets associated with individual differences in OUD susceptibility. As such, our studies promise to yield novel genomic and molecular targets for developing more effective, individualized approaches for the prevention and treatment of OUD.